Land Surveyor Forum

Replies

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  Billy Brooks March 9, 2017 at 12:21am

  I forgot to tell you there is even a electronic news section,just though I would tell you.

  The above article would be good to show in your class.

   To edit your post, above the title in red Edit This with a drop down option,you will see what you need.
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  Billy Brooks March 8, 2017 at 4:46pm

   Hi, William,

   I do not know if you use Science Daily ,I think you might like to read this since I gave this Post.

   It's kinda connected to yours.

  http://landsurveyorsunited.com/forum/topics/news-from-space-optical...

  https://www.sciencedaily.com/releases/2016/11/161117105146.htm

  I read from this site every day hope you like it.
• Not a Surveyor

  William H. Burling March 8, 2017 at 11:32am

  I can't figure out how to edit a post that I presented.

  So until I can figure it out,  i feel the explanatino above should be replaced by the explanation below.

  MY UNDERSTANDING OF CDMA

   

  In GPS the goal is to define an unknown location from known locations.

  To achieve that goal,  a satellite must transmit a timing reference pulse as well as additional data that reveals the exact time the satellite sent its data, the satellite’s ID, ephemeris data revealing the satellite’s exact orbit location for the time of transmission,  correction data and additional data.  

  This data is transmitted on a channel.  Each satellite has multiple channels at slightly different frequencies all of which redundantly transmit the same data.

  These multiple channels are common to a constellation of satellites. That is, for each unique channel(ie: L1) each satellite transmits on exactly the same frequency.

  The question arises as to how each satellite can convey its reference pulse and all its additional data without interfering with the data sent by any of the others if they are using the exact same frequency.

  The answer is actually quite easy if one is willing to and can send the data at a slower rate than the carrier frequency, yet preserve the precision and accuracy of the timing pulse.

  Here is how it is done. 

  Putting aside the details of a satellite transmission technology in which bits(a bit has the value of “zero” or “one”) are converted into waves of a given frequency and then back to bits) , is that these bits are carried by a carrier frequency.  One can imagine a carrier frequency as a string of square waves of exactly the same width and separation. Each single cycle of a square wave is a position for a bit.

  What is important is that a carrier frequency really defines the opportunity to store a bit.

  The key in transmitting multiple data groups on a single frequency is in appreciating that each data group (from each satellite) does not claim and use every bit storage opportunity of a carrier frequency.

  Lets label the position of every bit storage opportunity (corresponding to the carrier frequency)  from 1 to N.

  Satellite “1” might be assigned the use of bit position 1,6 9, 22, N, 17

  Satellite “2” might be assigned the use of bit position 2,11,33,47,N-5

  Satellite “3’ might be assigned the use of bit position 4,7,19,20,27

  Notice that no two satellites share the same bit position and hence data “orthgonality” or independence is attained. No information sent by one satellite will interferre with information from another satellite.The unique selection of bit positions for each satellite is known as a PRN code.

  Notice that to transmit “X bits” of the original data for a given satellite, the time it takes will be slower than if any one satellite could use all the bit positions of the carrier but as previously mentioned that time delay is not consequential. 

  Just as a reminder, the delay is not consequential because of two reasons.  The group of bits is sent with a time stamp (used ONLY to obtain a satellite’s orbit position at the time of the stamp)  AND the carrier’s pulse train consists of precise time intervals that are actually used in iteratively solving simultaneous equations to compute the delay in getting a satellite’s group of bits from satellite to receiver.

  I should mention that people often mention frequency hopping when they talk about CDMA.  Frequency hopping is, in effect, frequency modulation.  Ie, for a given group of bits, a bit position might be sent on frequency 1 and another on frequency 2.  The bit position vs frequency can be any set of combinations. Frequency hopping is generated at the satellite after the cdma code is created and frequency hopping decoded at the receiver before any cdma is decoded.